1. Field of the Invention
The present invention relates to motor-driven antennas for automobiles.
2. Prior Art
In conventional motorized antennas for automobiles, a rack rope, attached to the lower end of a rod antenna, is moved in a vertical direction by gears which are driven by a motor. In addition, a clutch is used between the motor for driving the antenna and the rack rope in order to minimize damage to the rack rope and other components when the rod antenna is locked.
When the power of a radio is turned on, the motor for the antenna starts to rotate, and the rod antenna is extended gradually. When the antenna reaches its maximum length, it is mechanically locked. When the rod antenna is thus locked, the current in the motor increases. Based upon such an increased motor current level, locking of the antenna is detected. At this moment, the clutch is disengaged, thereby interrupting the electric current to the motor.
In the above-described conventional apparatus, locking of the antenna is detected based on the electric current which is constantly flowing to the motor. If the motor current increases over a predetermined level, it is detected that the antenna has reached the fully extended maximum length and locked, and accordingly the motor current is shut off.
In the above-described conventional antenna, the disengaged clutch idles when the antenna is locked. During this idling period, the electric current in the motor does not flow steadily, and it is difficult to accurately monitor the current. Consequently, during the idling period, the clutch produces "clicking" noise. Also, idling wears both the clutch and the rack rope, lowering the reliability of the entire antenna system.